I got this exact question during Google onsite interview.

I don't see quad tree related questions very often so I decided to post it.

A quad tree is used to represent a black/white image. You are provided with two such image representations, write a function to create a third tree that represents the merged image. (Black overrides white, mixed; mixed overrides white).

Similar question: https://leetcode.com/problems/quad-tree-intersection/description/